Field of the Invention
The present invention relates to a control device and a control method.
Description of the Related Art
A three-phase brushless motor is known as one type of three-phase (a U phase, a V phase, and a W phase) motor. The three-phase brushless motor free from wear of brush is high in durability and widely used. In a general three-phase brushless motor, a rotation torque is generated by supplying a three-phase AC current to a three-phase coil. In recent years, it is possible to implement improvements in quietness and driving efficiency by performing current-drive control using a sine wave. Furthermore, vector control of controlling the amplitude and phase of a three-phase sinusoidal current precisely is becoming quite common.
In this vector control, the three-phase brushless motor is driven by pulse width modulation (to be referred to as PWM hereinafter) in which a switching element of an inverter undergoes an ON/OFF operation based on an instruction voltage for each phase. In the vector control, it is important to accurately detect currents flowing through three phases, respectively.
In general, a current sensor is used as a sensor that detects each of the currents flowing through the three phases. However, the current sensor is expensive, and thus not suitable for an embedded motor control device. Accordingly, in recent years, a method of using a shunt resistor connected in series with each switching element on an L side of an inverter and estimating each of the currents flowing through three phases from a voltage between terminals is used (see, for example, Japanese Patent Laid-Open No. 2008-48504).
In the method described in Japanese Patent Laid-Open No. 2008-48504, a PWM driving pattern in which a current flows through only one phase out of three phases is driven sequentially, and a current value is obtained from a voltage applied to the corresponding shunt resistor at that time. In such an arrangement, there is a restriction that the current value can be measured accurately only in a period during which the switching element on the L side is ON (period during which PWM is Low).